Repair a Broken Join
Select the safest first move when a join drops records.
Join failures are cleaning defects because keys encode assumptions. A join can fail from whitespace, casing, type mismatch, stale IDs, many-to-many relationships, missing bridge tables, or different entity grains. The safe move is to measure the join before fixing it: expected row counts, unmatched keys by side, duplicate keys by side, and revenue impact. This works because joins can create silent errors in both directions. Inner joins drop valid records. Many-to-many joins multiply revenue. Fuzzy joins can create false matches. A clean join is not one that runs; it is one whose unmatched and multiplied records are explained.
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